Self-attaching fasteners, including pierce and clinch nuts as disclosed in U.S. Pat. Nos. 2,707,322 and 3,648,747, assigned to the predecessor in interest of the assignee of this application, have been commercially successful in mass production applications, such as utilized by the automotive and appliance industries. The self-attaching female fastener elements disclosed in these patents are formed by rolling techniques including rolling a continuous strip of metal having the desired cross-section of the female fastener element including a rectangular central pilot portion and flange portions on opposed sides of the pilot portion. The pilot portion is then pierced through the end face, forming a bore of the fastener element, and the strip is then cut or chopped to length and the bore may then be internally threaded, as required. Reference is also made to U.S. Pat. Nos. 3,775,795 and 3,999,659, which disclose a method of forming a continuous strip of female fastener elements of the type disclosed in the above-referenced U.S. Pat. No. 2,707,322, wherein the rolled strip is only partially severed by transverse slots punched through the continuous pilot portion and a portion of the flanges, such that the strip may be severed in the installation head during installation of the female fastener elements in a panel.
As will be understood by those skilled in this art, the pilot portion formed by rolling a continuous strip must include a rectangular pilot portion having a rectangular end face. The rectangular end face of the self-attaching fastener is then utilized to pierce a rectangular opening in a panel and the pilot portion is then received through the pierced panel opening and attached to the panel by a die member or die button, as described below. Alternatively, the rectangular pilot portion is received through a preformed rectangular opening in the panel where the self-attaching fastener element is utilized as a clinch nut.
Self-attaching fastener elements of the type disclosed in the above-referenced patents are typically attached in a panel in a die press, wherein the upper die shoe or die platen includes an installation head, which receives the self-attaching fasteners and having a reciprocating plunger, and the lower die shoe or die platen includes a die member or die button which supports the panel during installation and installs the nut in the panel with each stroke of the die press. Because the panel may be simultaneously formed during installation of the self-attaching female fastener element, this method of installation is preferred in mass production applications. However, there are certain applications where a generally round or cylindrical pilot portion is preferred to avoid stress risers at the sharp corners of the rectangular panel opening. However, pierce or clinch nuts having a generally cylindrical or circular pilot portion are presently manufactured only by conventional cold forming or cold heading techniques. It would, however, be desirable to form self-attaching female fastener elements by continuously rolling a metal wire, thereby utilizing the advantages of this technology, but resulting in a self-attaching female fastener having a generally circular or cylindrical pilot portion, thereby achieving one of the advantages of a female fastener element formed by conventional cold heading techniques, including the elimination of stress risers. Further, self-attaching fasteners formed by cold forming are more expensive. The method of forming self-attaching female fastener elements of this invention achieves these advantages and the resultant female fastener element has advantages over the prior art as now described.